The invention relates to a process for control of an automatic transmission in which a downshift function is initiated with the actuation of a brake pedal.
A problem arises in automatic transmissions when, due to the increasing speed of the vehicle from downhill driving, upshifts are effected. This impairs the braking action of the motor. It is known from the prior art to suppress the upshifts. For example, DE-OS 31 39 985 proposes a process in which the reduction ratio of the automatic transmission is increased depending on the actuation of a brake pedal. It is, alternatively, proposed that the reduction ratio be adjusted so that the driving speed remains constant.
The invention solves the problem based on further development of the above described prior art.
According to the invention, the problem is solved by calculating in an initial downshift function a new downshift point, from a basic downshift point and a characteristic value K1. The characteristic value K1, in turn, is calculated from a driving activity and a calculated brake pressure. The brake pressure is calculated from the difference between an actual gradient and an imaginary gradient of the transmission output rotational speed. The imaginary gradient of the transmission output rotational speed is, in turn, determined by a characteristic field which represents a function of the vehicle speed and a torque reserve. The torque reserve is determined from the torque generated by the internal combustion engine and the resistance torques.
The inventive solution and developments thereof offer the advantage that the downshift function is better adapted to the environmental conditions. The torque reserve characterizes the road inclinations and, driving activity by the driver""s behavior, Since in the new generation of automatic transmissions with intelligent shift programs, shift points are selected depending on a driving activity, the inventive solution can thus refer to the already detected driving activity. The driving activity can be determined by a method, such as known from German Patent No. 39 22 051 (corresponding to U.S. Pat. No. 5,157,609) or DE-OS 39 41 999. According to German Patent No. 39 22 051 (U.S. Pat. No. 5,157,609), in which the driving speed, latest acceleration, longitudinal acceleration and deceleration, plus engine rotational speed, and throttle position are measured. These measurements being added together using a characteristic diagram and a linking function to form a parameter, can be further interpolated by filtering out a value for a driving activity. Said driving activity value is then used for the selection of a shifting program from a plurality of shifting programs ranging from a consumption-optimized program to a power-optimized program. Since in the new generation of automatic transmissions with intelligent shift programs, shift points are selected depending on the driving activity, the inventive solution can thus refer to the already detected driving activity. The inventive solution can be advantageously and economically integrated in existing software.
The downshift function terminates when the driver releases the brake pedal, the throttle valve value is higher than a minimum value, and a first delay time has elapsed. The minimum value is determined by a traction-push characteristic line. This development takes into account, e.g. the case at the end of an incline that the driver again communicates, via the throttle valve, a normal performance requirement.
In case of a significant change of the throttle valve signal evidenced by a large gradient during a first delay time, a second delay time is started when the significant change no longer exists and the downshift function terminates when the second delay time has expired. This development takes into account the case that, during the first delay time, the driver quickly actuates the acceleration pedal and then holds it constant. Accordingly, the downshift function terminates only with the expiration of the second delay time.
It is proposed that the second delay time again be started when the throttle valve is again actuated or the throttle valve gradient exceeds the limit value. The downshift function is then terminated independently of the further curve of the throttle valve when the second delay time has expired.